Carbon fiber precursor, method of producing carbon fiber precursor, method of producing stabilized fiber, and method of producing carbon fiber

ABSTRACT

The carbon fiber precursor contains a crosslinked acrylamide-based polymer and has a gel fraction of 5% or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC 119 from Japanese Patent Applications No. 2021-174074 filed on Oct. 25, 2021 and No. 2022-140764 filed on Sep. 5, 2022, the disclosure of which are incorporated by reference herein in its entirety.

BACKGROUND Technical Field

The present disclosure relates to a carbon fiber precursor, a method of producing the carbon fiber precursor, a method of producing a stabilized fiber, and a method of producing a carbon fiber.

Related Art

As the method of producing the carbon fiber, conventionally, a method of subjecting the carbon fiber precursor obtained by spinning polyacrylonitrile to stabilization treatment and then subjecting the carbon fiber precursor to carbonization treatment has been mainly employed (for example, Japanese Patent Publication (JP-B) No. S37-004405, and Japanese Patent Application Laid-Open (JP-A) Nos. 2015-074844, 2016-040419, and 2016-113726).

Since polyacrylonitrile is difficult to dissolve in an inexpensive general-purpose solvent, it is necessary to use an expensive organic solvent such as dimethyl sulfoxide or N,N-dimethylacetamide when performing polymerization, spinning, and the like, and there has been a problem that the production cost of the carbon fiber increases.

On the other hand, an acrylamide-based polymer containing an acrylamide-based monomer is a water-soluble polymer, and water can be used as a solvent, which is inexpensive and has a low environmental impact, so that it can be expected to reduce the production cost of the carbon material.

Since the carbon fiber precursor containing the acrylamide-based polymer has a softening temperature and a cyclization temperature close to each other at the time of stabilization treatment, fiber fusion may occur. In addition, the acrylamide-based polymer has high hygroscopicity, and in the carbon fiber precursor containing the acrylamide-based polymer, single fibers may be fused to each other during storage, stabilization treatment, or the like.

When the single fibers are fused to each other, the stabilization treatment and the subsequent carbonization treatment do not proceed sufficiently, and there is a possibility that a carbonization yield and a tensile strength of the stabilized fiber are reduced. In addition, there is a possibility that a surface of the carbon fiber is roughened due to peeling of a fused portion, and the carbon fiber has a defect.

For the purpose of suppressing fusion between the single fibers and improving the carbonization yield, JP-A No. 2021-080610 proposes a method of producing a stabilized fiber, including subjecting an acrylamide-based polymer fiber made of an acrylamide/vinyl cyanide/unsaturated carboxylic acid-based copolymer containing 40 to 99.8 mol % of an acrylamide-based monomer unit, 0.1 to 50 mol % of a vinyl cyanide-based monomer unit, and 0.1 to 30 mol % of an unsaturated carboxylic acid-based monomer unit, to the stabilization treatment while applying a tension of 0.05 to 200 mN/tex.

SUMMARY OF THE INVENTION

As in JP-A No. 2021-080610 described above, attempts have been heretofore made to suppress the fusion between the single fibers and improve the carbonization yield, but further improvement in fusion suppression is required. Further, since the acrylamide-based polymer fiber easily absorbs moisture, it is also required to improve moisture absorption inhibiting properties, and shrinkage inhibiting properties during storage.

An object of an embodiment of the present disclosure is to provide a carbon fiber precursor, a method of producing the carbon fiber precursor, a method of producing a stabilized fiber, and a method of producing a carbon fiber, which are excellent in suppression of fusion between fibers, capable of improving the tensile strength of the stabilized fiber without reducing the carbonization yield, and excellent in moisture absorption inhibiting properties, and shrinkage inhibiting properties during storage.

Specific means for achieving the object are as follows.

<1> A carbon fiber precursor containing a crosslinked acrylamide-based polymer and having a gel fraction of 5% or more.

<2> The carbon fiber precursor according to the above <1>, in which the gel fraction is 98% or less.

<3> The carbon fiber precursor according to the above <1> or <2>, in which the crosslinked acrylamide-based polymer contains 30 mol % or more of an acrylamide-based monomer unit.

<4> The carbon fiber precursor according to any one of the above <1> to <3>, in which the crosslinked acrylamide-based polymer is a copolymer of an acrylamide-based monomer, a vinyl cyanide-based monomer, and an unsaturated carboxylic acid.

<5> A method of producing a carbon fiber precursor, including irradiating an acrylamide-based polymer fiber with active ray to crosslink an acrylamide-based polymer contained in the acrylamide-based polymer fiber.

<6> The method of producing the carbon fiber precursor according to the above <5>, in which the acrylamide-based polymer contains 30 mol % or more of an acrylamide-based monomer unit.

<7> The method of producing the carbon fiber precursor according to the above <5> or <6>, in which the acrylamide-based polymer is a copolymer of an acrylamide-based monomer, a vinyl cyanide-based monomer, and an unsaturated carboxylic acid.

<8> A method of producing a stabilized fiber, including subjecting the carbon fiber precursor according to any one of the above <1> to <4> to heat treatment in an oxidizing atmosphere.

<9> A method of producing a carbon fiber, including subjecting the stabilized fiber produced by the method of producing the stabilized fiber according to the above <8> to carbonization treatment.

According to the present disclosure, it is possible to provide a carbon fiber precursor, a method of producing the carbon fiber precursor, a method of producing a stabilized fiber, and a method of producing a carbon fiber, which are excellent in suppression of fusion between fibers, moisture absorption inhibiting properties, and shrinkage inhibiting properties during storage, and can improve tensile strength of a stabilized fiber without reducing carbonization yield.

DETAILED DESCRIPTION OF THE INVENTION

In the present disclosure, a numerical range indicated using “to” includes numerical values described before and after “to” as a minimum value and a maximum value, respectively.

In numerical ranges described in stages in the present disclosure, an upper limit value or a lower limit value described in one numerical range may be replaced with an upper limit value or a lower limit value of a numerical range described in another stage. In addition, in the numerical range described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with a value shown in a synthesis example.

In the present disclosure, each of components may contain a plurality of applicable substances. When a plurality of substances corresponding to each component are present in the carbon fiber precursor, content rate or content of each component means content rate or content based on a total of the plurality of substances present in the carbon fiber precursor unless otherwise specified.

In the present disclosure, the “carbon fiber precursor” means a fiber that can be subjected to carbonization treatment, or stabilization treatment and the carbonization treatment to obtain carbon fiber.

In the present disclosure, a “crosslinked acrylamide-based polymer” means a polymer obtained by crosslinking an acrylamide-based polymer by irradiation with active ray, heating or the like.

In the present disclosure, the “acrylamide-based polymer” means a homopolymer of an acrylamide-based monomer or a copolymer of an acrylamide-based monomer and a monomer (hereinafter, referred to as another polymerizable monomer) other than the acrylamide-based monomer.

In the present disclosure, the “active energy rays” means a ray capable of crosslinking the acrylamide-based polymer, and examples thereof include α rays, β rays, γ rays, X rays, neutron rays, electron rays, ultraviolet rays, and infrared rays. Among them, it is preferable to use the electron ray because it is easy to adjust depth, intensity, and the like.

<Carbon Fiber Precursor>

The carbon fiber precursor of the present disclosure (hereinafter, also simply referred to as the carbon fiber precursor) contains the crosslinked acrylamide-based polymer, and has a gel fraction of 5% or more.

The carbon fiber precursor of the present disclosure is excellent in suppression of fusion between fibers, moisture absorption inhibiting properties, and shrinkage inhibiting properties during storage, and can improve tensile strength of a stabilized fiber without reducing carbonization yield.

A reason why the above effect is obtained is assumed as follows, but is not limited thereto.

Since the carbon fiber precursor of the present disclosure contains the crosslinked acrylamide-based polymer and has a gel fraction of 5% or more, hygroscopicity is reduced due to crosslinking density thereof, so that it is possible to suppress fusion between single fibers and shrinkage of the fiber during storage, stabilization treatment, and the like.

When the stabilization treatment is performed, cyclization reaction in the stabilization treatment is satisfactorily performed, so that it is assumed that the tensile strength of the stabilized fiber is improved without reducing the carbonization yield. In addition, it is assumed that the tensile strength of the carbon fiber is also improved by improving the tensile strength of the stabilized fiber.

In addition, since it is possible to suppress fusion between carbon fibers during storage, a burden of temperature control during storage of the carbon fiber precursor tends to be reduced.

In addition, it is assumed that since the carbon fiber precursor of the present disclosure contains the crosslinked acrylamide-based polymer and has a gel fraction of 5% or more, it is possible to suppress shrinkage of the carbon fiber precursor during storage, and is excellent in shape stability.

The carbon fiber precursor may be a single fiber or a fiber bundle.

From the viewpoint of fusion inhibiting properties, carbonization yield, and shape stability, the gel fraction of the carbon fiber precursor is preferably 5% or more, more preferably 10% or more, even more preferably 15% or more, still even more preferably 20% or more, particularly preferably 30% or more, and most preferably 50% or more.

The gel fraction of the carbon fiber precursor is preferably 98% or less, more preferably 95% or less, and even more preferably 90% or less. By setting the gel fraction of the carbon fiber precursor to 98% or less, in a drawing treatment of the carbon fiber precursor that can be included in a method of producing the stabilized fiber described later, orientation of the crosslinked acrylamide-based polymer is improved, and the tensile strength of the stabilized fiber tends to be improved.

In the present disclosure, the gel fraction is measured by the following method.

First, 0.1 g of a sample is cut out from the carbon fiber precursor, dried at 90° C. for 2 hours, and then precisely weighed using a precision electronic balance to obtain an initial mass (g). As the precision electronic balance, for example, AUW220D manufactured by Shimadzu Corporation can be used.

Next, the sample is immersed in 30 ml of deionized water and left in a hot air circulating oven at 90° C. for 2 hours.

The sample after standing is subjected to suction filtration using a membrane filter having a pore size of 1.0 μm to separate a gel component. Residue remaining on the membrane filter without being dissolved in deionized water corresponds to the gel component. As the membrane filter, for example, an Omnipore™ membrane filter JAWP04700 manufactured by Merck KGaA can be used.

The separated gel component is left together with the membrane filter in the hot air circulating oven at 90° C. for 12 hours to remove moisture.

The mass of the gel component and the membrane filter after standing are precisely weighed using the precision electronic balance, and the gel fraction is determined from the following formula.

Gel fraction (%)=(mass (g) of gel component and membrane filter−mass (g) of membrane filter)/initial mass (g) of sample×100

When the carbon fiber precursor is the fiber bundle, the number of filaments per bundle is not particularly limited, but is preferably 50 to 96000, more preferably 100 to 48000, even more preferably 500 to 36000, and particularly preferably 1000 to 24000, from the viewpoint of productivity and mechanical properties of the stabilized fiber and the carbon fiber.

In addition, by setting the number of filaments per bundle to 96000 or less, occurrence of uneven calcination during the stabilization treatment can be suppressed.

Fineness of the carbon fiber precursor is not particularly limited, but is preferably 1×10⁻⁸ tex/fiber to 100 tex/fiber, more preferably 1×10⁻⁶ tex/fiber to 60 tex/fiber, even more preferably 1×10⁻³ tex/fiber to 40 tex/fiber, still even more preferably 1×10⁻² tex/fiber to 10 tex/fiber, particularly preferably 2×10⁻² tex/fiber to 5 tex/fiber, and most preferably 3×10⁻² tex/fiber to 3 tex/fiber.

By setting the fineness of the carbon fiber precursor to 1×10⁻⁸ tex/fiber or more, occurrence of yarn breakage can be suppressed, so that ease of winding the carbon fiber precursor and stability of the stabilization treatment tend to be improved.

By setting the fineness of the carbon fiber precursor to 100 tex/fiber or less, a difference between a structure near a surface layer and a structure near a center of the carbon fiber obtained by the stabilization treatment can be reduced, and the tensile strength and a tensile modulus of the carbon fiber tend to be improved.

In the present disclosure, in measurement of single fiber fineness (tex/fiber), 100 carbon fiber precursors are bundled to prepare the fiber bundle, the mass of the fiber bundle is measured, and the single fiber fineness is determined by the following formula.

Single fiber fineness (tex)=mass (g) of fiber bundle/fiber length (m)×1000/100 (yarns)

An average fiber diameter of the carbon fiber precursors is not particularly limited, but is preferably 3 nm to 300 μm, more preferably 30 nm to 250 μm, even more preferably 1 μm to 200 μm, particularly preferably 3 μm to 100 μm, even particularly preferably 4 μm to 40 μm, most preferably 5 μm to 30 μm, and may be 6 μm to 20 μm.

By setting the average fiber diameter of the carbon fiber precursors to 3 nm or more, the stability of the stabilization treatment tends to be improved. In addition, by setting the average fiber diameter of the carbon fiber precursors to 3 nm or more, the occurrence of yarn breakage can be suppressed, and this tends to improve the ease of winding the carbon fiber precursor and the stability of the stabilization treatment.

By setting the average fiber diameter of the carbon fiber precursors to 300 μm or less, the difference between the structure near the surface layer and the structure near the center of the carbon fiber obtained by the stabilization treatment can be reduced, and the tensile strength and the tensile modulus of the carbon fiber tend to be improved.

In the present disclosure, 100 carbon fiber precursors are bundled to prepare the fiber bundle, a density of the fiber bundle is measured using a dry automatic densitometer, and an average fiber diameter of the single fibers constituting the fiber bundle is determined by the following formula. Note that as the dry automatic densitometer, AccuPyc II 1340 manufactured by Micromeritics or a device similar thereto can be used.

D={(Dt×4×1000)/(ρ×π×n)}^(1/2)

In the formula,

D represents an average fiber diameter (μm) of the single fibers constituting the fiber bundle,

Dt represents fineness (tex) of the fiber bundle,

ρ represents a density (g/cm³) of the fiber bundle, and

n represents the number of single fibers constituting the fiber bundle. π is 3.14.

The carbon fiber precursor of the present disclosure may have a surface applied with a conventionally known oil agent.

Since the oil agent is applied to the surface of the carbon fiber precursor, bundling properties and handling of the fibers can be improved, and the fusion of the single fibers can be prevented.

In addition, by crosslinking the oil agent together with the acrylamide-based polymer, the fusion of the single fibers can be more effectively prevented.

As the oil agent, a silicone-based oil agent can be used. In addition, as the oil agent, an oil agent having a functional group to be crosslinked by irradiation with active ray and/or heat or the like is preferred, a silicone-based oil agent having the functional group is more preferred, and a silicone-based oil agent having a functional group to be crosslinked by irradiation with active ray is particularly preferred.

(Crosslinked Acrylamide-Based Polymer)

The carbon fiber precursor of the present disclosure contains the crosslinked acrylamide-based polymer. Further, the carbon fiber precursor of the present disclosure may contain two or more kinds of crosslinked acrylamide-based polymers.

The crosslinked acrylamide-based polymer may be the homopolymer of the acrylamide-based monomer, or may be the copolymer of the acrylamide-based monomer and the monomer (hereinafter, referred to as another polymerizable monomer) other than the acrylamide-based monomer.

From the viewpoint of the fusion inhibiting properties, the carbonization yield, the shape stability, the tensile strength of the stabilized fiber, and the like, the crosslinked acrylamide-based polymer is preferably the copolymer of the acrylamide-based monomer and another polymerizable monomer.

The content rate of the acrylamide-based monomer unit in the crosslinked acrylamide-based polymer is preferably 30 mol % or more, more preferably 40 mol % or more, even more preferably 50 mol % or more, particularly preferably 55 mol % or more, and most preferably 60 mol % or more.

When the content rate of the acrylamide-based monomer unit is 30 mol % or more, solubility of the acrylamide-based polymer before crosslinking in an aqueous solvent or an aqueous mixed solvent tends to be improved.

Further, an upper limit of the content rate of the acrylamide-based monomer unit is not particularly limited, but is preferably 99.9 mol % or less, more preferably 99 mol % or less, even more preferably 95 mol % or less, particularly preferably 90 mol % or less, and most preferably 85 mol % or less, from the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability.

When the crosslinked acrylamide-based polymer is the copolymer of the acrylamide-based monomer and other polymerizable monomer, the content of other polymerizable monomer units in the copolymer is preferably 0.1 mol % or more, more preferably 1 mol % or more, even more preferably 5 mol % or more, particularly preferably 10 mol % or more, and most preferably 15 mol % or more, from the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability.

In addition, from the viewpoint of improving the solubility of the acrylamide-based polymer before crosslinking in the aqueous solvent or the aqueous mixed solvent, an upper limit of the content rate of other polymerizable monomer units is preferably 70 mol % or less, more preferably 60 mol % or less, even more preferably 50 mol %, particularly preferably 45 mol % or less, and most preferably 40 mol % or less.

Examples of the acrylamide-based monomer include: acrylamide; ethacryl amide; crotonamide; itaconic acid diamide; cinnamic acid amide; maleic acid diamide; N-alkylacrylamides such as N-methylacrylamide, N-ethylacrylamide, N-n-propylacrylamide, N-isopropylacrylamide, N-n-butylacrylamide, and N-tert-butylacrylamide; N-cycloalkylacrylamide such as N-cyclohexylacrylamide; dialkylacrylamides such as N,N′-dimethylacrylamide; dialkylaminoalkylacrylamides such as dimethylaminoethylacrylamide and dimethylaminopropylacrylamide; hydroxyalkyl acrylamides such as N-(hydroxymethyl) acrylamide and N-(hydroxyethyl) acrylamide; N-arylacrylamide such as N-phenylacrylamide; diacetone acrylamide; N,N′-alkylene bisacrylamide such as N,N′-methylene bisacrylamide; methacrylamide; N-alkyl methacrylamide such as N-methyl methacrylamide, N-ethyl methacrylamide, N-n-propyl methacrylamide, N-isopropyl methacrylamide, N-n-butyl methacrylamide, and N-tert-butyl methacrylamide; N-cycloalkylmethacrylamide such as N-cyclohexylmethacrylamide; dialkyl methacrylamide such as N,N-dimethylmethacrylamide; dialkylaminoalkyl methacrylamide such as dimethylaminoethyl methacrylamide and dimethylaminopropyl methacrylamide; hydroxyalkyl methacrylamide such as N-(hydroxymethyl) methacrylamide and N-(hydroxyethyl) methacrylamide; N-aryl methacrylamide such as N-phenyl methacrylamide; diacetone methacrylamide; and N,N′-alkylene bismethacrylamide such as N,N′-methylene bismethacrylamide.

Further, from the viewpoint of the solubility of the acrylamide-based polymer before crosslinking in the aqueous solvent or the aqueous mixed solvent, among the acrylamide-based monomers described above, acrylamide, N-alkylacrylamide, dialkylacrylamide, methacrylamide, N-alkylmethacrylamide, or dialkylmethacrylamide is preferred, and acrylamide is more preferred.

One kind of the acrylamide-based monomer may be used alone, or two or more kinds thereof may be used in combination.

Note that the carbon fiber precursor of the present disclosure may contain one or more acrylamide-based monomers.

Examples of other polymerizable monomers include vinyl cyanide-based monomers, unsaturated carboxylic acids and salts thereof, unsaturated carboxylic acid anhydrides, unsaturated carboxylic acid esters, vinyl alcohol-based monomers, vinyl carboxylate-based monomers, and olefin-based monomers.

Examples of the vinyl cyanide-based monomer include acrylonitrile, methacrylonitrile, 2-hydroxyethylacrylonitrile, chloroacrylonitrile, chloromethylacrylonitrile, ethoxyacrylonitrile, and vinylidene cyanide.

Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, crotonic acid, and isocrotonic acid.

Examples of the salt of the unsaturated carboxylic acid include metal salts (for example, sodium salt, potassium salt, and the like), ammonium salts, and amine salts of the unsaturated carboxylic acid.

Examples of the unsaturated carboxylic acid anhydride include maleic acid anhydride and itaconic acid anhydride.

Examples of the unsaturated carboxylic acid ester include methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate.

Examples of the vinyl-based monomer include aromatic vinyl-based monomers such as styrene and α-methylstyrene, vinyl chloride, and vinyl alcohol.

Examples of the olefin-based monomer include ethylene, propylene, isopropylene, and butadiene.

Among the other polymerizable monomers described above, from the viewpoint of spinnability, the fusion inhibiting properties, the carbonization yield, and the shape stability of the acrylamide-based polymer, the vinyl cyanide-based monomer is preferred, and acrylonitrile is more preferred.

Further, among the other polymerizable monomers described above, from the viewpoint of the solubility of the copolymer before crosslinking in the aqueous solvent or the aqueous mixed solvent, unsaturated carboxylic acid and the salt thereof are preferred, and acrylic acid, maleic acid, fumaric acid, or itaconic acid is more preferred.

Furthermore, among the other polymerizable monomers described above, from the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability, the unsaturated carboxylic acid or the unsaturated carboxylic acid anhydride is preferred, and acrylic acid, maleic acid, fumaric acid, itaconic acid, or maleic acid anhydride is more preferred.

One kind of the other polymerizable monomer described above may be used alone, or two or more kinds thereof may be used in combination.

From the viewpoint of the solubility of the copolymer before crosslinking in the aqueous solvent or the aqueous mixed solvent, the spinnability, the fusion inhibiting properties, the carbonization yield, and the shape stability, the crosslinked acrylamide-based polymer is particularly preferably a copolymer of the acrylamide-based monomer, the vinyl cyanide-based monomer, and the unsaturated carboxylic acid, and is most preferably a copolymer of acrylamide, acrylonitrile, and acrylic acid.

From the viewpoint of the spinnability, the fusion inhibiting properties, the carbonization yield, and the shape stability, the content rate of the vinyl cyanide-based monomer unit in the copolymer is preferably 1 mol % to 50 mol %, and more preferably 10 mol % to 40 mol %.

From the viewpoint of the solubility of the copolymer before crosslinking in the aqueous solvent or the aqueous mixed solvent, the fusion inhibiting properties, the carbonization yield, and the shape stability, the content rate of the unsaturated carboxylic acid unit in the copolymer is preferably 0.1 mol % to 40 mol %, and more preferably 1 mol % to 10 mol %.

Since a preferred numerical range of the content rate of the acrylamide-based monomer unit in the copolymer has been described above, description thereof is omitted here.

From the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability, the content rate of the crosslinked acrylamide-based polymer based on the mass of the carbon fiber precursor of the present disclosure and/or an acrylamide-based polymer fiber is preferably 50 mass % or more, more preferably 80 mass % or more, even more preferably 90 mass % or more, particularly preferably 95 mass % or more, even particularly preferably 99 mass % or more, and most preferably 100 mass %.

In an infrared absorption spectrum of the crosslinked acrylamide-based polymer, an infrared absorption peak is preferably observed in a range of about 1644 cm⁻¹ to 1653 cm⁻¹.

The infrared absorption peak is an absorption peak derived from a stretching motion of a carbonyl group in the acrylamide-based monomer unit.

Note that the infrared absorption spectrum is measured using infrared spectroscopy. Specifically, the infrared absorption spectrum is measured by an attenuated total reflection (ATR) method in which measurement range is 400 cm⁻¹ to 4000 cm⁻¹, resolution is 0.482 m⁻¹, and the number of integrations is 32.

As a measuring device, for example, a Fourier transform infrared spectrometer Nicolet 8700FT-IR manufactured by Thermo Scientific, or a device similar thereto can be used.

A weight average molecular weight of the acrylamide-based polymer before crosslinking is not particularly limited, and is usually 5 million or less, but is preferably 2 million or less, more preferably 1 million or less, even more preferably 500,000 or less, particularly preferably 200,000 or less, even particularly preferably 130,000 or less, and most preferably 100,000 or less, from the viewpoint of molding processability of the carbon fiber precursor.

Further, a lower limit of the weight average molecular weight of the acrylamide-based polymer is not particularly limited, and is usually 10,000 or more, but is preferably 20,000 or more, more preferably 30,000 or more, and particularly preferably 40,000 or more, from the viewpoint of strength of the carbon fiber precursor and the carbon fiber.

Note that in the present disclosure, the weight average molecular weight is measured by gel permeation chromatography under the following conditions. As a measuring device, HLC-8220GPC manufactured by Tosoh Corporation or a device similar thereto can be used.

Measurement conditions

-   -   Column: TSKgel GMPWXL×2 columns+TSKgel G2500PWXL×1 column     -   Eluent: 100 mM sodium nitrate aqueous solution/acetonitrile         (=80/20 (volume ratio))     -   Eluent flow rate: 1.0 ml/min     -   Column temperature: 40° C.     -   Molecular weight standard substance: standard polyethylene         oxide/standard polyethylene glycol     -   Detector: differential refractive index detector

(Additive Component)

The carbon fiber precursor of the present disclosure can contain at least one additive component selected from the group consisting of an acid and a salt thereof.

Since the carbon fiber precursor of the present disclosure is excellent in the fusion inhibiting properties, the carbonization yield, and the shape stability, it is not necessary to add the additive component such as the acid, but the carbon fiber precursor may contain at least one additive component selected from the group consisting of the acid and the salt thereof in addition to the crosslinked acrylamide-based polymer. By subjecting the carbon fiber precursor containing the additive component to the stabilization treatment, formation of a cyclic structure by a dehydration reaction, a deammoniation reaction, or the like is accelerated, and the fusion inhibiting properties, the carbonization yield, and the shape stability tend to be further improved.

Further, in the stabilized fiber, at least a part of the additive component and residue thereof may remain. Furthermore, the carbonization treatment may be performed by adding the additive component to the stabilized fiber.

Examples of the acid include inorganic acids such as phosphoric acid, polyphosphoric acid, boric acid, sulfuric acid, nitric acid, and carbonic acid, and organic acids such as oxalic acid, citric acid, and sulfonic acid.

Examples of the salt of the acid include metal salts (sodium salt, potassium salt, and the like), ammonium salts, and amine salts, and guanidine salt, and urea salt, and imidazole salt, and the ammonium salts and the amine salts are preferred, and the ammonium salts are more preferred.

Among the above-mentioned additive components, from the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability, phosphoric acid, polyphosphoric acid, boric acid, and sulfuric acid, or ammonium salts thereof are preferred, phosphoric acid, polyphosphoric acid, and boric acid, or ammonium salts thereof are more preferred, and phosphoric acid and polyphosphoric acid, or ammonium salts thereof are even more preferred.

The content of the additive component with respect to 100 parts by mass of the crosslinked acrylamide-based polymer contained in the carbon fiber precursor of the present disclosure is preferably 0.1 parts by mass to 100 parts by mass, more preferably 0.2 parts by mass to 50 parts by mass, even more preferably 0.5 parts by mass to 30 parts by mass, and particularly preferably 1 part by mass to 20 parts by mass, from the viewpoint of the carbonization yield, the fusion inhibiting properties, and the shape stability.

<Method of Producing Carbon Fiber Precursor>

A method of producing the carbon fiber precursor includes irradiating the acrylamide-based polymer fiber with active ray to crosslink the acrylamide-based polymer contained in the acrylamide-based polymer fiber.

Further, the method of producing the carbon fiber precursor of the present disclosure includes applying an oil agent having a functional group to be crosslinked by irradiation with active ray and/or an oil agent having no functional group to be crosslinked by irradiation with active ray to a surface of the acrylamide-based polymer fiber, and irradiating the oil agent with active ray to crosslink the oil agent. Note that the crosslinking of the oil agent may be performed simultaneously with the crosslinking of the acrylamide-based polymer.

The carbon fiber precursor may contain a low molecular weight compound to be crosslinked with active ray, such as N-vinylacetamide, a vinyl acetate monomer, vinylethoxysilane, methacrylic acid, 2-isocyanatoethyl methacrylate, N-vinyl-2 pyrrolidone, N-vinyl-2 caprolactam, triethylene glycol divinyl ether, ethylene dimethacrylate, divinylbenzene, or triallyl isocyanurate, as long as the effect is not impaired.

From the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability, a dose of the active ray irradiated to the acrylamide-based polymer fiber is preferably 1 kGy to 10,000 kGy, more preferably 100 kGy to 5000 kGy, and even more preferably 150 kGy to 1000 kGy.

Note that the above-mentioned preferred numerical range of the dose is a preferred numerical range of the dose when the acrylamide-based polymer fiber is irradiated with active ray from one direction, and when the acrylamide-based polymer fiber is irradiated from two or more directions, the numerical range is not limited to the above, and is preferably appropriately adjusted.

When an electron beam is used as the active ray, the dose is measured by using a film dosimeter. As the film dosimeter, an FWT-60 type manufactured by Toyo Medic Co., Ltd. or a device similar thereto can be used.

From the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability, an acceleration voltage of the active ray irradiated to the acrylamide-based polymer fiber is preferably adjusted to an acceleration voltage at which preferably 20% or more, more preferably 60% or more, and even more preferably 80% or more of the active ray irradiated is transmitted through the acrylamide-based polymer fiber.

When the electron beam is used as the active ray, transmittance of the electron beam is calculated by measuring the dose before and after transmission. In addition, it may be calculated from a relationship diagram between a transmission depth and a relative dose that is generally disclosed.

Specifically, the acceleration voltage is preferably 50 kV to 10 MV, more preferably 100 kV to 3 MV, and even more preferably 150 kV to 1 MV.

Note that the above-mentioned preferred numerical range of the acceleration voltage is a preferred numerical range of the acceleration voltage when the acrylamide-based polymer fiber is irradiated with active ray from one direction, and when the acrylamide-based polymer fiber is irradiated from two or more directions, the numerical range is not limited to the above, and is preferably appropriately adjusted.

The irradiation with active ray may be performed by a batch method or a continuous method.

A device used for the irradiation with active ray is not particularly limited, but when the irradiation with active ray by the batch method is performed, CB250/30/20 mA manufactured by Iwasaki Electric Co., Ltd. or a device similar thereto can be used. When the irradiation with active ray by the continuous method is performed, an electron beam irradiation device EBC800-35 manufactured by NHV Corporation or a device similar thereto can be used.

As the acrylamide-based polymer fiber to be used in the method of producing the carbon fiber precursor of the present disclosure, an acrylamide-based polymer fiber produced by a conventionally known method may be used.

The acrylamide-based polymer fiber can be produced by spinning an acrylamide-based polymer composition containing the acrylamide-based polymer or the acrylamide-based polymer, the additive component, and the like.

A spinning method is not particularly limited, and may be performed by, for example, melt-spinning, spunbonding, melt-blowing, or centrifugally spinning a molten acrylamide-based polymer or acrylamide-based polymer composition.

Further, when the acrylamide-based polymer or the acrylamide-based polymer composition is soluble in the aqueous solvent or the aqueous mixed solvent, from the viewpoint of the spinnability, environmental impact reduction, cost, and safety, it is preferred that the acrylamide-based polymer or the acrylamide-based polymer composition is dissolved in the aqueous solvent or the aqueous mixed solvent, and spinning is performed using the obtained aqueous solution or aqueous mixed solution to produce the acrylamide-based polymer fiber.

Further, when the acrylamide-based polymer is synthesized by solution polymerization, it is preferred that a solution of the acrylamide-based polymer is adjusted to a desired content rate as necessary and then spun to produce the acrylamide-based polymer fiber.

Furthermore, when the acrylamide-based polymer composition is produced by wet mixing, it is preferred that the solution of the acrylamide-based polymer composition is adjusted to the desired content rate as necessary and then spun to produce the acrylamide-based polymer fiber.

The spinning is preferably performed by a dry spinning method, a wet spinning method, a dry-wet spinning method, a gel spinning method, a flash spinning method, or an electrospinning method. According to the spinning method, the acrylamide-based polymer fiber having desired fineness and average fiber diameter can be safely produced at low cost.

In addition, from the viewpoint that the acrylamide-based polymer fiber can be safely produced at a lower cost, it is preferable to use the aqueous solvent as the solvent, and it is more preferable to use water.

As the acrylamide-based polymer, the commercially available acrylamide-based polymer may be used, or an acrylamide-based polymer produced by a conventionally known method may be used.

The acrylamide-based polymer can be synthesized by using a known polymerization reaction such as radical polymerization, cationic polymerization, anionic polymerization, or living radical polymerization. Among the above polymerization reactions, radical polymerization is preferred from the viewpoint of reducing synthesis cost.

Further, the acrylamide-based polymer can be synthesized by using a polymerization method such as solution polymerization, suspension polymerization, precipitation polymerization, dispersion polymerization, or emulsion polymerization (for example, inverse emulsion polymerization).

Further, when the acrylamide-based polymer is synthesized by solution polymerization, it is preferable to use, as the solvent, a solvent in which a monomer of a raw material and the acrylamide-based polymer to be obtained are dissolved, and it is more preferable to use the aqueous solvent or the aqueous mixed solvent, and it is even more preferable to use the aqueous solvent, from the viewpoint of safe synthesis at low cost.

Examples of the aqueous solvent include water, alcohols, and mixed solvents thereof, and water is particularly preferable.

The aqueous mixed solvent means a mixed solvent of the aqueous solvent and the organic solvent, and examples of the organic solvent include tetrahydrofuran.

In the synthesis of the acrylamide-based polymer by radical polymerization, it is preferable to use a polymerization initiator.

As the polymerization initiator, conventionally known radical polymerization initiators such as azobisisobutyronitrile, benzoyl peroxide, 4,4′-azobis(4-cyanovaleric acid), ammonium persulfate, and potassium persulfate can be used.

When the aqueous solvent or the aqueous mixed solvent is used as the solvent, the radical polymerization initiator such as 4,4′-azobis(4-cyanovaleric acid), ammonium persulfate, or potassium persulfate, that is soluble in the aqueous solvent or the aqueous mixed solvent, is preferred.

In addition, from the viewpoint of controlling the molecular weight of the acrylamide-based polymer and improving the spinnability of the acrylamide-based polymer, it is preferable to use at least one of a polymerization accelerator or a molecular weight modifier in place of or together with the polymerization initiator, and it is more preferable to use the polymerization initiator and the polymerization accelerator in combination.

Examples of the polymerization accelerator include tetramethylethylenediamine.

Examples of the molecular weight modifier include alkyl mercaptan compounds such as n-dodecyl mercaptan.

It is particularly preferable to use ammonium persulfate that is the polymerization initiator and tetramethylethylenediamine that is the polymerization accelerator in combination.

A temperature of the polymerization reaction is not particularly limited, but is preferably 35° C. or higher, more preferably 40° C. or higher, even more preferably 50° C. or higher, particularly preferably 70° C. or higher, and most preferably 75° C. or higher, from the viewpoint of improving the spinnability of the acrylamide-based polymer.

The acrylamide-based polymer may be the homopolymer of the acrylamide-based monomer, or may be the copolymer of the acrylamide-based monomer and another polymerizable monomer. Since a preferred aspect of the acrylamide-based monomer and other polymerizable monomers have been described above, description thereof is omitted here.

The content rate of the acrylamide-based monomer unit in the acrylamide-based polymer is preferably 30 mol % or more, more preferably 40 mol % or more, even more preferably 50 mol % or more, particularly preferably 55 mol % or more, and most preferably 60 mol % or more.

When the content rate of the acrylamide-based monomer unit is 30 mol % or more, the solubility in the aqueous solvent or the aqueous mixed solvent tends to be improved.

Further, an upper limit of the content rate of the acrylamide-based monomer unit is not particularly limited, but is preferably 99.9 mol % or less, more preferably 99 mol % or less, even more preferably 95 mol % or less, particularly preferably 90 mol % or less, and most preferably 85 mol % or less, from the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability.

When the acrylamide-based polymer is the copolymer of the acrylamide-based monomer and other polymerizable monomer, the content of other polymerizable monomer unit in the copolymer is preferably 0.1 mol % or more, more preferably 1 mol % or more, even more preferably 5 mol % or more, particularly preferably 10 mol % or more, and most preferably 15 mol % or more, from the viewpoint of the fusion inhibiting properties, the carbonization yield, and the shape stability.

In addition, from the viewpoint of improving the solubility of the acrylamide-based polymer in the aqueous solvent or the aqueous mixed solvent, the upper limit of the content rate of another monomer unit is preferably 70 mol % or less, more preferably 60 mol % or less, even more preferably 50 mol %, particularly preferably 45 mol % or less, and most preferably 40 mol % or less.

Examples of the method of producing the acrylamide-based polymer composition include a method of directly mixing the additive component with the molten acrylamide-based polymer (melt mixing), a method of dry mixing the acrylamide-based polymer and the additive component (dry mixing), or a method of immersing or passing the acrylamide-based polymer formed into fibers in the aqueous solution or the aqueous mixed solution containing the additive component, or a solution in which the acrylamide-based polymer is not completely dissolved, but the additive component is dissolved.

Further, when the acrylamide-based polymer and the additive component are soluble in the aqueous solvent or the aqueous mixed solvent, from the viewpoint that the acrylamide-based polymer and the additive component can be uniformly mixed, a method of mixing the acrylamide-based polymer and the additive component in the aqueous solvent or the aqueous mixed solvent (wet mixing) is preferred.

Furthermore, the wet mixing may be performed by mixing the additive component in the aqueous solvent or the aqueous mixed solvent in which the acrylamide-based polymer has been synthesized.

In addition, in the wet mixing, from the viewpoint that the acrylamide-based polymer composition can be safely produced at a lower cost, it is preferable to use the aqueous solvent as the solvent, and it is more preferable to use water.

Note that when the acrylamide-based polymer composition is produced by wet mixing, the solvent may or may not be removed. A method of removing the solvent is not particularly limited, and at least one of known methods such as distillation under reduced pressure, reprecipitation, hot air drying, vacuum drying, or freeze drying can be used.

<Method of Producing Stabilized Fiber>

The method of producing the stabilized fiber of the present disclosure includes subjecting the above-mentioned carbon fiber precursor to the stabilization treatment. The stabilization treatment refers to subjecting the carbon fiber precursor to heat treatment in an oxidizing atmosphere. The carbon fiber precursor produced by the above production method can be used.

The stabilization treatment is not particularly limited, but is preferably carried out at a temperature in a range of 150° C. to 500° C., more preferably at a temperature in a range of 200° C. to 450° C., and even more preferably at a temperature in a range of 250° C. to 420° C.

Note that the temperature includes not only a maximum temperature (stabilization treatment temperature) at the time of stabilization treatment described later but also a temperature in a temperature raising process or the like up to a stabilization treatment temperature.

The maximum temperature (stabilization treatment temperature) during the stabilization treatment is preferably 250° C. to 500° C., more preferably 280° C. to 450° C., even more preferably 290° C. to 420° C., still even more preferably 300° C. to 400° C., particularly preferably 305° C. to 390° C., and most preferably 310° C. to 380° C.

By setting the stabilization treatment temperature to 250° C. or higher, heat resistance and the carbonization yield of the stabilized fiber tend to be improved.

Further, by setting the stabilization treatment temperature to 500° C. or lower, production cost tends to be reduced.

In the stabilization treatment of the carbon fiber precursor, it is preferable to subject the carbon fiber precursor to the drawing treatment. By subjecting the carbon fiber precursor to the drawing treatment, the crosslinked acrylamide-based polymer contained in the carbon fiber precursor is oriented, and the tensile strength of the stabilized fiber tends to be increased.

The drawing treatment is preferably performed at least during heating at the stabilization treatment temperature.

Further, from the viewpoint of increasing the tensile strength of the stabilized fiber, it is preferred that the drawing treatment is also performed in the temperature raising process up to the stabilization treatment temperature.

Furthermore, the drawing treatment may be performed while controlling moisture absorptivity in a spinning treatment process or a pre-process of the stabilization treatment.

A tension applied to the carbon fiber precursor during the drawing treatment is preferably 0.03 mN/tex to 2000 mN/tex, more preferably 0.05 mN/tex to 500 mN/tex, even more preferably 0.07 mN/tex to 200 mN/tex, and particularly more preferably 0.1 mN/tex to 100 mN/tex.

Note that in the present disclosure, the tension (unit: mN/tex) applied to the carbon fiber precursor is a value obtained by dividing the tension (unit: mN) applied to the carbon fiber precursor during the stabilization treatment by the fineness (unit: tex) of the carbon fiber precursor in an absolutely dry state, that is, a tension per unit fineness of the carbon fiber precursor.

In addition, the tension can be adjusted by adjusting a speed at an inlet and an outlet of a heating device such as a stabilized furnace or using a load cell, a spring, a weight, an air cylinder, or the like.

The stabilization treatment time (heating time at the stabilization treatment temperature) is not particularly limited, but is preferably 1 minute to 120 minutes, more preferably 2 minutes to 60 minutes, even more preferably 3 minutes to 50 minutes, and particularly preferably 4 minutes to 40 minutes, from the viewpoint of the carbonization yield and the production cost.

Note that the stabilization treatment time may be set to a long time exceeding 2 hours.

A density of the stabilized fiber produced by the above-mentioned production method is not particularly limited, but is preferably 1.30 g/cm³ to 1.75 g/cm³, more preferably 1.35 g/cm³ to 1.70 g/cm³, even more preferably 1.37 g/cm³ to 1.65 g/cm³, particularly preferably 1.39 g/cm³ to 1.60 g/cm³, and most preferably 1.44 g/cm³ to 1.55 g/cm³, from the viewpoint of the carbonization yield, the productivity, and the like.

The average fiber diameter of the stabilized fibers is not particularly limited, but is preferably 3 nm to 300 μm, more preferably 30 nm to 150 μm, even more preferably 1 μm to 60 μm, particularly preferably 2 μm to 30 μm, and most preferably 3 μm to 20 μm, and may be 5 μm to 15 μm, from the viewpoint of the tensile strength of the resulting carbon fiber.

From the viewpoint of the carbonization yield, the average fiber diameter of the stabilized fibers is preferably 5% or more smaller than the average fiber diameter of the carbon fiber precursors, more preferably 10% or more smaller, even more preferably 15% or more smaller, particularly preferably 20% or more smaller, and most preferably 25% or more smaller, and may be 30% or more smaller than the average fiber diameter of the carbon fiber precursors.

<Method of Producing Carbon Fiber>

The method of producing the carbon fiber of the present disclosure includes subjecting the stabilized fiber obtained by the method of producing the stabilization fiber to the carbonization treatment. The carbonization treatment refers to a treatment for carbonizing the carbon fiber precursor, and means that the heat treatment is performed under a low oxygen environment (preferably an environment in which oxygen is blocked).

Examples of a carbonization treatment method include a method of heat-treating the stabilized fiber at a temperature higher than a temperature in the stabilization treatment under an inert gas (nitrogen, argon, helium, or the like) atmosphere.

By the carbonization treatment, the stabilized fiber is carbonized to obtain the carbon fiber.

A heating temperature in the carbonization treatment is preferably 500° C. or higher, more preferably 1000° C. or higher, even more preferably 1100° C. or higher, particularly preferably 1200° C. or higher, and most preferably 1300° C. or higher.

Further, the upper limit value of the heating temperature is preferably 3000° C. or lower, more preferably 2500° C. or lower.

Furthermore, the heating time in the carbonization treatment is not particularly limited, but is preferably 30 seconds to 60 minutes, and more preferably 1 minute to 30 minutes.

Note that in the present disclosure, the “carbonization treatment” may include “graphitization” performed by heating generally at a temperature of 2000° C. to 3000° C. under the inert gas atmosphere.

Further, the carbonization treatment may include a plurality of times of heat treatment.

For example, it is possible to first perform heat treatment (preliminary carbonization treatment) at a temperature of lower than 1000° C., then perform heat treatment (carbonization treatment) at a temperature of 1000° C. or higher, and further perform heat treatment (graphitization treatment) at a temperature of 2000° C. or higher.

The average fiber diameter of the carbon fibers produced by the production method described above is not particularly limited, but is preferably 3 nm to 300 μm, more preferably 30 nm to 150 μm, even more preferably 1 μm to 60 μm, still even more preferably 2 μm to 20 μm, particularly preferably 3 μm to 15 μm, and most preferably 5 μm to 10 μm, from the viewpoint of the tensile strength.

EXAMPLES

Hereinafter, the above-described embodiments will be specifically described with reference to examples, but the above-described embodiments are not limited to these examples.

<Single Fiber Fineness of Acrylamide-Based Polymer Fiber>

In the present example, the single fiber fineness of the acrylamide-based polymer fiber was calculated by preparing the fiber bundle by bundling 100 of the obtained acrylamide-based polymer fibers, measuring the mass of the fiber bundle, and calculating the single fiber fineness (tex) by the following formula.

Single fiber fineness (tex)=mass (g) of fiber bundle/fiber length (m)×1000/100 (yarns)

<Average Fiber Diameter of Acrylamide-Based Polymer Fibers>

In the present example, the average fiber diameter of the acrylamide-based polymer fibers was determined by bundling 100 of the obtained acrylamide-based polymer fibers to prepare the fiber bundle, measuring the density (g/cm³) of the fiber bundle using the dry automatic densitometer (“AccuPyc II 1340” manufactured by Micromeritics), and determining the average fiber diameter (μm) of the single fibers constituting the fiber bundle by the following formula.

D={(Dt×4×1000)/(ρ×π×n)}^(1/2)

In the formula,

D represents an average fiber diameter (μm) of the single fibers constituting the fiber bundle,

Dt represents fineness (tex) of the fiber bundle,

ρ represents a density (g/cm³) of the fiber bundle, and

n represents the number of single fibers constituting the fiber bundle. π is 3.14.

Preparation Example 1

100 parts by mass of a composition containing 62 mol % of acrylamide (AM), 35 mol % of acrylonitrile (AN), and 3 mol % of acrylic acid (AA), and 4.4 parts by mass of tetramethylethylenediamine were dissolved in 566.7 parts by mass of deionized water to obtain an aqueous solution.

After 3.4 parts by mass of ammonium persulfate was added to the obtained aqueous solution with stirring under a nitrogen atmosphere, the solution was heated at 70° C. for 150 minutes, then heated to 90° C. over 30 minutes, and then heated at 90° C. for 1 hour to perform a polymerization reaction.

The obtained aqueous solution was dropped into methanol to precipitate a copolymer, and the copolymer was recovered and vacuum-dried at 80° C. for 12 hours to obtain a water-soluble AM/AN/AA copolymer.

Production Example 1

The AM/AN/AA copolymer (AM/AN/AA=62 mol %/35 mol %/3 mol %) obtained in Preparation Example 1 was dissolved in deionized water to obtain an aqueous solution.

Dry spinning was performed using the obtained aqueous solution to prepare acrylamide-based polymer fibers (100 fibers/bundle) having a single fiber fineness of 0.54 tex/fiber and an average fiber diameter of 23 μm.

Example 1

The acrylamide-based polymer fiber (100 fibers/bundle) obtained in Production Example 1 was fixed to a mount.

The fixed acrylamide-based polymer fiber was subjected to a batch type electron beam irradiation treatment using an electron beam irradiation device CB250/30/20 mA manufactured by Iwasaki Electric Co., Ltd. under conditions of a conveyance speed of 5 m/min, a nitrogen purge, an acceleration voltage of 250 kV, and an electron beam dose of 150 kGy to obtain a carbon fiber precursor (100 fibers/bundle) containing a crosslinked acrylamide-based polymer.

A temperature during the electron beam irradiation treatment was 14° C., and relative humidity was 20%.

Note that the mount to which the acrylamide-based polymer fiber was fixed was stored in an aluminum laminate bag containing a silica gel desiccant and taken out before the electron beam irradiation treatment and used. After the electron beam irradiation treatment, the mount was stored in the aluminum laminate bag containing the silica gel desiccant.

About 0.1 g of a sample was cut out from the carbon fiber precursor obtained as described above, and dried at 90° C. for 2 hours, then the mass of the carbon fiber precursor was accurately weighed using a precision electronic balance (AUW220D manufactured by Shimadzu Corporation), and taken as an initial mass (g).

Subsequently, the sample was immersed in 30 ml of deionized water and left in a hot air circulating oven at 90° C. for 2 hours, and then a gel component was separated by suction filtration using a membrane filter having a pore size of 1.0 μm. Residue remained on the membrane filter, and thus it was found that the gel component was present.

The separated gel component was left together with the membrane filter in the hot air circulating oven at 90° C. for 12 hours to remove moisture.

After removal of moisture, total mass (g) of the gel component and the membrane filter was measured, and a gel fraction was calculated from the following formula, and found to be 34%.

Gel fraction (%)=(mass (g) of gel component and membrane filter−mass (g) of membrane filter)/initial mass (g) of sample×100

Example 2

A carbon fiber precursor (100 fibers/bundle) was obtained in the same manner as in Example 1 except that the electron beam dose was changed to 200 kGy. As in Example 1, the gel fraction was measured and found to be 63%.

Example 3

A carbon fiber precursor was obtained in the same manner as in Example 1 except that the electron beam irradiation treatment was performed by the following method. As in Example 1, the gel fraction was measured and found to be 48%.

3200 of the acrylamide-based polymer fibers obtained in Production Example 1 were bundled, and subjected to a continuous electron irradiation treatment in the atmosphere (air temperature: 21° C., relative humidity: 45%) using an electron beam irradiation device EBC800-35 manufactured by NHV Corporation under conditions of a conveyance speed of 10 m/min, a feed tension of 70 g, a winding tension of 500 g, an acceleration voltage of 800 kV, and an electron beam dose of 100 kGy to obtain a carbon fiber precursor (3200 fibers/bundle).

Examples 4 to 6

Carbon fiber precursors of Examples 4 to 6 were obtained in the same manner as in Example 3 except that the electron beam dose was changed to 300 kGy, 600 kGy, and 1100 kGy.

As in Example 1, the gel fractions were measured and found to be respectively 51%, 74%, and 86%.

Comparative Example 1

The acrylamide-based polymer fiber (100 fibers/bundle) obtained in Production Example 1 was used as a carbon fiber precursor. As in Example 1, the gel fraction was measured and found to be 0%.

<<Measurement of Moisture Absorptivity>>

About 0.1 mg of each of the carbon fiber precursors of Examples 1 to 6 and Comparative Example 1 was cut out, put in a weighing bottle, and dried at 90° C. for 2 hours, and then the mass of the carbon fiber precursor was accurately weighed using a precision electronic balance (AUW220D manufactured by Shimadzu Corporation), and taken as an initial mass (g).

The weighing bottle containing the carbon fiber precursor was set with a lid opened in a stopped thermo-hygrostat.

The thermo-hygrostat was started, and left for 4 hours in an environment of a temperature of 85° C. and a relative humidity of 50%.

After standing, the weighing bottle was taken out from the thermo-hygrostat, cooled to room temperature (25° C.), and the total mass of the carbon fiber precursor and the weighing bottle was measured.

The moisture absorptivity was determined by the following formula and summarized in Table 1.

Moisture absorptivity (%)=(mass (g) of carbon fiber precursor after standing−initial mass (g) of carbon fiber precursor)/initial mass (g) of carbon fiber precursor×100

The moisture absorptivity of the carbon fiber precursors of Examples 1 to 6 were respectively 5.7%, 6.0%, 5.5%, 9.7%, 5.2%, and 4.5%.

The moisture absorptivity of the carbon fiber precursor of Comparative Example 1 was 31.0%.

<<Measurement of Shrinkage Rate of Carbon Fiber Precursor>>

A fiber bundle having a length of 50 mm was cut out from each of the carbon fiber precursors of Examples 1 to 6 and Comparative Example 1, and was left for 12 hours in a thermo-hygrostat set at an environmental condition (temperature: 30° C., relative humidity: 95%) assuming that the carbon fiber precursor is left at room temperature in rainy weather.

The fiber bundle of the carbon fiber precursor after standing was taken out from the thermo-hygrostat, and the length of the fiber bundle was measured.

A shrinkage rate of the carbon fiber precursor was determined by the following formula and summarized in Table 1.

Shrinkage rates of the fiber bundles of the carbon fiber precursors of Examples 1 to 6 were respectively 33%, 34%, 71%, 40%, 40%, and 30%.

The shrinkage rate of the fiber bundle of the carbon fiber precursor of Comparative Example 1 was 74%.

Shrinkage rate (%)=(length (mm) of fiber bundle of carbon fiber precursor before standing−length (mm) of fiber bundle of carbon fiber precursor after standing)/length (mm) of fiber bundle of carbon fiber precursor before standing×100

<<Measurement of Fusion Rate>>

The carbon fiber precursors of Example 1 were bundled to make 200 fibers/bundle.

The carbon fiber precursor was placed in a heating furnace, and heated from room temperature (25° C.) to 350° C. at a rate of 10° C./min in an air atmosphere (dew point temperature: −17° C.) to be subjected to heat treatment (stabilization treatment) and obtain stabilized fibers. Temperature rise of the carbon fiber precursor was performed while applying a tension of 0.4 mN/tex.

In the stabilization treatment, the stabilization treatment temperature was 350° C. (maximum temperature during the stabilization treatment), and the stabilization treatment time was 30 minutes.

An evaluation fiber (200 fibers/bundle) having a length of 2 cm was cut out from the stabilized fiber obtained by the stabilization treatment, a cross-section of the evaluation fiber was observed using a microscope (“digital microscope VHX-7000” manufactured by Keyence Corporation), the number of fibers was counted, and a ratio of the carbon fiber precursor before the stabilization treatment to the number of fibers was calculated as a fusion rate.

Fusion rates of Examples 2 to 6 and Comparative Example 1 were similarly collected and summarized in Table 1.

Note that in Examples 3 to 6, the carbon fiber precursor (3200/fiber) was bundled to give 6400 fibers/bundle.

The fusion rates of Examples 1 to 6 were respectively 30%, 54%, 59%, 45%, 32%, and 26%.

The fusion rate of Comparative Example 1 was 78%.

<<Measurement of Carbonization Yield>>

The carbon fiber precursors of Examples 1 to 6 and Comparative Example 1 were heated from room temperature (25° C.) to 350° C. under a condition of a temperature raising rate of 10° C./min in an air atmosphere with an air flow rate of 1000 ml/min, and held at 350° C. for 10 minutes (stabilization treatment) to obtain stabilized fibers.

Using a differential thermal balance (“Thermo Plus EVO2 TG-DTA8122/H” manufactured by Rigaku Corporation), the stabilized fiber was heated (carbonized) from room temperature (25° C.) to 1050° C. under a condition of a temperature rising rate of 20° C./min in a nitrogen atmosphere with a nitrogen flow rate of 1000 ml/min to obtain a carbon fiber.

The carbonization yield was determined by the following formula and summarized in Table 1.

Note that as the mass of the stabilized fiber before carbonization treatment, the stabilized fiber was vacuum-dried at 150° C. for 2 hours to calculate an amount of moisture adsorbed to the stabilized fiber, and a value in consideration of the amount of moisture was used.

The carbonization yields of Examples 1 to 6 were respectively 69%, 74%, 67%, 71%, 66%, and 65%.

The carbonization yield of Comparative Example 1 was 63%.

Carbonization yield (%)=mass (g) of carbon fiber/mass (g) of stabilization fiber before carbonization treatment×100

<<Measurement of Tensile Strength of Stabilized Fiber>>

The carbon fiber precursors of Examples 2 to 6 and Comparative Example 1 were heated from room temperature (25° C.) to 350° C. under a condition of a temperature raising rate of 10° C./min in an air atmosphere with a flow rate of 1000 ml/min, and held at 350° C. for 10 minutes (stabilization treatment) to obtain stabilized fibers (100 fibers/bundle).

Single fibers were taken out from the obtained stabilized fibers, and subjected to a tensile test (gauge length: 25 mm, tensile speed: 1 mm/min) at room temperature (25° C.) in accordance with JIS R 7606:2000 using a micro-strength evaluation tester, tensile strength (MPa) was measured, and an average value of 5 times was calculated and summarized in Table 1.

Note that a fiber diameter of the single fiber was measured by using a digital microscope VHX-1000 manufactured by Keyence Corporation at four random points, and a minimum value among them was taken as the fiber diameter.

Tensile strengths of the stabilized fibers of Examples 2 to 6 were respectively 253 MPa, 240 MPa, 245 MPa, 290 MPa, and 207 MPa.

The tensile strength of the stabilized fiber of Comparative Example 1 was 151 MPa.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 1 Acrylamide-based polymer fiber type AM/AN/AA copolymer (AM/AN/AA = 62 mol %/35 mol %/3 mol %) Electron Treatment method Batch method Continuous method — irradiation Acceleration voltage (kV) 250 250 800 800 800 800 — condition Electron beam dose (kGy) 150 200 100 300 600 1100 — Gel fraction of carbon fiber precursor (%) 34 63 48 51 74 86 0 Evaluation Moisture absorptivity of 5.7 6.0 5.5 9.7 5.2 4.5 31.0 carbon fiber precursor (%) Shrinkage rate of carbon 33 34 71 40 40 30 74 fiber precursor (%) Fusion rate (%) 30 54 59 45 32 26 78 Carbonization yield (%) 69 74 67 71 66 65 63 Tensile strength of — 253 240 245 290 207 151 stabilized fiber (MPa)

From Table 1, it can be understood that the carbon fiber precursors of Examples containing a crosslinked acrylamide-based polymer and having a gel fraction of 5% or more are excellent in moisture absorption inhibiting properties, shrinkage inhibiting properties during storage, fusion inhibiting properties, and tensile strength of the stabilized fiber while maintaining a good carbonization yield, as compared with the carbon fiber precursor of Comparative Example not containing a crosslinked acrylamide-based polymer and having a gel fraction of less than 5%. 

What is claimed is:
 1. A carbon fiber precursor, comprising a crosslinked acrylamide-based polymer and having a gel fraction of 5% or more.
 2. The carbon fiber precursor according to claim 1, wherein the gel fraction is 98% or less.
 3. The carbon fiber precursor according to claim 1, wherein the crosslinked acrylamide-based polymer contains 30 mol % or more of an acrylamide-based monomer unit.
 4. The carbon fiber precursor according to claim 1, wherein the crosslinked acrylamide-based polymer is a copolymer of an acrylamide-based monomer, a vinyl cyanide-based monomer, and an unsaturated carboxylic acid.
 5. A method of producing a carbon fiber precursor, the method comprising irradiating an acrylamide-based polymer fiber with an active ray to crosslink an acrylamide-based polymer contained in the acrylamide-based polymer fiber.
 6. The method of producing the carbon fiber precursor according to claim 5, wherein the acrylamide-based polymer contains 30 mol % or more of an acrylamide-based monomer unit.
 7. The method of producing the carbon fiber precursor according to claim 5, wherein the acrylamide-based polymer is a copolymer of an acrylamide-based monomer, a vinyl cyanide-based monomer, and an unsaturated carboxylic acid.
 8. A method of producing a stabilized fiber, comprising subjecting the carbon fiber precursor according to claim 1 to heat treatment in an oxidizing atmosphere.
 9. A method of producing a carbon fiber, the method comprising subjecting the flame resistant fiber produced by the method of producing the stabilized fiber according to claim 8 to carbonization treatment. 